Parameters are measured for the 5 standard filters UBVRI_BESSELL and 4port read-out (high gain for all filters, low gain only for R_BESSELL).
QC1 parameters

conversion factor (1/gain) (QC1 database table fors1_scrflat_1CCD, column conad)
The conversion factor is measured in a raw file (high gain mode) as 100x100 pixels sigma, corrected for fixed-pattern contribution. The sigma is compared to the square-root of the signal averaged across 4 ports. The number given is in e-/ADU and is averaged across all 4 ports. It is trended only for the R_BESSELL filter.

relative fixed-pattern noise (QC1 database table fors1_scrflat_1CCD, column FPN_rel)
A derivative frame of a master M is created (by shifting a copied version M' by 10 pixels in X and Y so that fixed-pattern noise is randomized, and calculating the difference frame M -M' ). It is divided by a background fit. A 100x100 pixels sigma is measured. The photon noise is subtracted geometrically from that number. Fixed-pattern noise should scale with signal. It is trended only for the R_BESSELL filter.

contamination (QC1 database table fors1_scrflat_1CCD, column contam)
This parameter measures a change of large-scale gradients across port A (by comparing to a reference frame), after removal of small-scale contributions. This way the introduction of additional structure in the flats (e.g. due to dust) should be detected.

History

CCD Contamination was present in the old FORS1 CCD at the beginning of operations. This time dependent effect is clearly visible when computing the ratio of two Imaging Screen Flats taken at two different epochs. In the following example, two U Master Screen Flats taken one just after a decontamination process (1999-11-02) and the other just before the next intervention (2000-01-27) have been used to compute the ratio.
At the beginning of FORS1 operations the contamination was growing very fast and this required very close in time calibrations in order to properly remove it from the science frames.

When the contamination was judged to be too high, the detector was usually baked. Such operation was performed on a periodical basis and this was reflected into a saw-tooth behavior of the contamination amount. A detailed description of contamination measurements is given in contam_report_fors1.ps (400kB).